CCNA – STP Questions 2
Here you will find answers to STP Questions – Part 2
Note: If you are not sure about STP, please read my STP tutorial and RSTP tutorial.
Question 1
Which term describes a spanning-tree network that has all switch ports in either the blocking or forwarding state?
A. redundant
B. spanned
C. provisioned
D. converged
Answer: D
Explanation
Spanning Tree Protocol convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged, root bridge is elected and all port roles (Root, Designated and Non-Designated) in all switches are selected.
Question 2
Which two values are used by Spanning Tree Protocol to elect a root bridge? (Choose two)
A. amount of RAM
B. bridge priority
C. IOS version
D. IP address
E. MAC address
F. speed of the links
Answer: B E
Explanation
Bridge ID = Bridge Priority + MAC Address
For example:
+ The bridge priority of SwA is 32768 and its MAC address is 0000.0000.9999 -> the bridge ID of SwA is 32768:0000.0000.9999
+ The bridge priority of SwB is 32768 and its MAC address is 0000.0000.1111 -> the bridge ID of SwB is 32768:0000.0000.1111
Question 3
Which command enhances the 802.1D convergence time on ports that are connected to hosts?
A. spanning-tree backbonefast
B. spanning-tree uplinkfast
C. spanning-tree portfast
D. spanning-tree cost512
Answer: C
Explanation
By using PortFast feature, the port won’t spend 50 seconds to move from blocking (20sec), listening (15sec), learning (15sec) and finally forwarding but will jump directly to the forwarding state. This feature should be used on ports connected to hosts only because hosts surely don’t send BPDU. An example of configuring PortFast on an interface is shown below:
Sw(config)#interface FastEthernet0/1
Sw(config-if)#spanning-tree portfast
Question 4
Which two of these are used by bridges and switches to prevent loops in a layer 2 network? (Choose two)
A. 802.1D
B. VTP
C. 802.1Q
D. STP
E. SAP
Answer: A D
Explanation
Both 802.1D and STP are referred to the STP standard which is used by bridges and switches to prevent loops in a layer 2 network.
Question 5
Refer to the exhibit. The output that is shown is generated at a switch. Which three of these statements are true? (Choose three)
A. All ports will be in a state of discarding, learning or forwarding.
B. Thirty VLANs have been configured on this switch.
C. The bridge priority is lower than the default value for spanning tree.
D. All interfaces that are shown are on shared media.
E. All designated ports are in a forwarding state.
F. The switch must be the root bridge for all VLANs on this switch.
Answer: A C E
Explanation
From the output, we see that all ports are in Designated role (forwarding state) -> A and E are correct.
The command “show spanning-tree vlan 30″ only shows us information about VLAN 30. We don’t know how many VLAN exists in this switch -> B is not correct.
The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768 -> C is correct.
All three interfaces on this switch have the connection type “p2p”, which means Point-to-point environment – not a shared media -> D is not correct.
The only thing we can specify is this switch is the root bridge for VLAN 3o but we can not guarantee it is also the root bridge for other VLANs -> F is not correct.
Question 6
What is one benefit of PVST+?
A. PVST+ reduces the CPU cycles for all the switches in the network.
B. PVST+ automatically selects the root bridge location, to provide optimization.
C. PVST+ allows the root switch location to be optimized per vlan.
D. PVST+ supports Layer 3 load balancing without loops.
Answer: C
Explanation
Per VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It means a switch can be the root bridge of a VLAN while another switch can be the root bridge of other VLANs in a common topology. For example, Switch 1 can be the root bridge for Voice data while Switch 2 can be the root bridge for Video data. If designed correctly, it can optimize the network traffic.
Question 7
Which IEEE standard protocol is initiated as a result of successful DTP completion in a switch over FastEthernet?
A. 802.3ad
B. 802.1w
C. 802.1Q
D. 802.1d
Answer: C
Explanation
Dynamic Trunking Protocol (DTP) is a Cisco proprietary protocol for negotiating trunking on a link between two devices and for negotiating the type of trunking encapsulation (802.1Q) to be used.
Question 8
What value is primarily used to determine which port becomes the root port on each non-root switch in a spanning-tree topology?
A. lowest port MAC address
B. port priority number and MAC address.
C. VTP revision number
D. highest port priority number.
E. path cost
Answer: E
Explanation
The path cost to the root bridge is the most important value to determine which port will become the root port on each non-root switch. In particular, the port with lowest cost to the root bridge will become root port (on non-root switch).
Question 9
When PVST+ in work on VLAN1 of the switch, what will affect on selection of one of switches in the VLAN as root-bridge?
A. Lowest IP address
B. Highest MAC address
C. Lowest MAC address
D. Highest IP address
Answer: C
Explanation
The two values used to select the root bridge are Bridge Priority & MAC address (the lower is better) -> C is correct.
Question 10
Refer to the exhibit. A network administrator wants Switch3 to be the root bridge. What could be done to ensure Switch3 will be the root?
A. Configure the IP address on Switch3 to be higher than the IP addresses of Switch1 and Switch2.
B. Configure the priority value on Switch3 to be higher than the priority values of Switch 1 and Switch2.
C. Configure the BID on Switch3 to be lower than the BIDs of Switch1 and Switch2.
D. Configure the MAC address on Switch3 to be higher than the Switch1 and Switch2 MAC addresses.
E. Configure a loopback interface on Switch3 with an IP address lower than any IP address on Switch1 and Switch2.
Answer: C
Explanation
To become root bridge, a switch must have lower Bridge ID (BID) than that of the others. The Bridge ID = Bridge Priority + MAC address; but MAC address is a fixed value so we can only change the BID by changing the Bridge Priority of that switch.
Question 11
Which port state is introduced by Rapid-PVST?
A. learning
B. listening
C. discarding
D. forwarding
Answer: C
Explanation
PVST+ is based on IEEE802.1D Spanning Tree Protocol (STP). But PVST+ has only 3 port states (discarding, learning and forwarding) while STP has 5 port states (blocking, listening, learning, forwarding and disabled). So discarding is a new port state in PVST+.
Question 12
At which layer of the OSI model is RSTP used to prevent loops?
A. data link
B. network
C. physical
D. transport
Answer: A
Explanation
RSTP and STP operate on switches and are based on the exchange of Bridge Protocol Data Units (BPDUs) between switches. One of the most important fields in BPDUs is the Bridge Priority in which the MAC address is used to elect the Root Bridge -> RSTP operates at Layer 2 – Data Link layer -> A is correct.
Question 13
Refer to the exhibit. At the end of an RSTP election process, which access layer switch port will assume the discarding role?
A. Switch3, port fa0/1
B. Switch3, port fa0/12
C. Switch4, port fa0/11
D. Switch4, port fa0/2
E. Switch3, port Gi0/1
Answer: C
Explanation
In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.
Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior than another if it has:
1. A lower Root Bridge ID
2. A lower path cost to the Root
3. A lower Sending Bridge ID
4. A lower Sending Port ID
These four parameters are examined in order. In this specific case, all the BPDUs sent by Switch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 will compare port index values, which are unique to each port on the switch, and because Fa0/12 is inferior to Fa0/1, Switch4 will select the port connected with Fa0/1 (of Switch3) as its root port and block the other port -> Port fa0/11 of Switch4 will be blocked (discarding role).
If you are still not sure about this question, please read my RSTP tutorial.
Q5 F should be correct instead of A, when we apply root primary command, if the current root’s priority is higher that 24576 –> it makes the priority to 24576 on the SW that we applied the command, in this case 24576+30=24606, so based on this fact we could assume that F is correct answer.
Mah, see line 8 of the output.
Valid Que :D
Q:3
Q:9
No, mah. That answer implies that the switch will be the bridge for -ALL- VLANs. Given the output, we have absolutely no way of knowing which switch is the bridge for whatever other VLANs are configured (if there even are any). For all we know, Switch8 has been configured with “spanning-tree vlan 40 root primary,” or some infinite other number of possibilities. There’s no way to know, which makes the statement not true for lack of knowledge. A, C, E are the best answers, although I dislike A’s wording…Cisco has an annoying obsession with using both vague and absolute terms far too often in their tests.
Q5, anyone can confirm which is the correct answer? in pass4sure it states C,E,F are the correct answers….. and A is wrong as the the switch is utilizing RSTP, the new RSTP states are Disable,Blocking, Listening,Learning and Forwarding….. I am confused now which is the correct answer!
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VALID Q
Q#9
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pls, could 9tut tell me if in the exam certification if one question has multiple answer and I select two well and one bad, does they take this completly as bad or does they give me some points to my favor side.
pls, could someone tell me if in the exam certification if one question has multiple answer and I select two well and one bad, does they take this completly as bad or does they give me some points to my favor side.
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Question 5 / Q5:
A. All ports will be in a state of discarding, learning or forwarding.
F. The switch must be the root bridge for all VLANs on this switch.
We are dealing with the situation under VLAN 30 here.
In general for RSTP “A” is correct but since we are dealing with VLAN 30 this is wrong.
Now if we look at “F”, since the Priority is lower than the default (correct answer “C”), we might assume that this “must be” the root bridge for all VLANs.
If “F” is rephrased to (The switch “is” the root bridge for all VLANs on this switch) then this would be wrong.
This one is a bit tricky, for this is only my analysis of the situation. I am open to all ideas for we might encounter this kind of situation in the exam.
Feel free to comment for we all benefit for any information shared.
Thanks!
I’ve been looking for a post like this forveer (and a day)
Q5) F should not be the correct instead of A
F. The switch must be the root bridge for all VLANs on this switch.
With PVRST, you can control the root assignment to a Switch as you wish for each VLAN with the below commands
Switch (config)# spanning-tree vlan 1 root primary
Switch (config)# spanning-tree vlan 1 priority 4096 ; lower value
switch# show spanning-tree vlan 30
…
Spanning tree enabled protocol rstp
…..
Here, “rstp” means, it runs the “Per Vlan Rapid Spanning Tree”
Please advice the Question 5 on the 9tut.com the answer are A C E but on the exam collection the answer are C E F
witch is correct one URGENT Please
Q5:
Answer F, there is VERB ” MUST BE ” which means WRONG. It`s due to your configuration on SW.
Q5:
I tnink the answer is A C E.
A – All ports will be in a state of discarding, learning or forwarding.it’s a general sentence about RSTP (Rapid STP – listening, blocking is eliminated)
F – How we know that the switch is root switch for all vlans ?
@ocal
You are right and answers A C and E are correct.
About questionon F need to explain shortly that if you use command ” SHOW SPANNING-TREE ” on SWITCH then you would see the result about all current VLAN situation which is not show in this question totally.
Hi all.
Q5: On my opinion the correct answers should be A C E.
According to W.Odom’s book RSTP protocol has only three port states: Discarding, Learning and Forwarding, so A makes sence.
Also the output shows RSTP information for vlan 30 only. We can assume that #spanning-tree vlan 30 root primary command had been applied, but it has to do with certain vlan (vlan 30) only, not with every vlan configured on the SW.
Please advice the
Question 5 on the 9tut.com the answer are A C E
but on the exam collection spike the answer are C E F
witch is correct one URGENT Please ??????/
tomorrow is my exam
Please advice the
Question 5 on the 9tut.com the answer are A C E
but on the exam collection spike the answer are C E F
witch is correct one URGENT Please ??????/
my exam
next week
q12 in today exam
Question 5, answer F is flat out wrong.
A C E is correct
Pliz i will sit for the CCNA exam next month, can someone send me the latest dumps on email: olesimbe@yahoo.com
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Q5 looks right to me, in that the answer should be A C E. My reasoning for saying that F is wrong is that the question uses the word MUST, but part of rstp is per-VLAN spanning tree configuration, meaning that Switch could in fact be the root bridge for VLAN30, but not for other VLANs. Below are the commands and output to simulate this in my lab.
=== SwitchA is currently not root bridge ====
SwitchA#show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol rstp
Root ID Priority 32798
Address 0019.e7db.d100
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 0023.ea8d.cd00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Altn BLK 4 128.26 P2p
=== Let’s make SwitchA root bridge for vlan 30 ===
SwitchA#conf t
Enter configuration commands, one per line. End with CNTL/Z.
SwitchA(config)#spanning-tree vlan 30 priority 24576
SwitchA(config)#end
SwitchA#
*Mar 1 00:16:00.579: %SYS-5-CONFIG_I: Configured from console by console
SwitchA#show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol rstp
Root ID Priority 24606
Address 0023.ea8d.cd00
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24606 (priority 24576 sys-id-ext 30)
Address 0023.ea8d.cd00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–
Gi0/1 Desg FWD 4 128.25 P2p
Gi0/2 Desg FWD 4 128.26 P2p
=== SwitchA now root for vlan 30, but not for other vlan that was left on default ===
SwitchA#show spanning-tree vlan 2
VLAN0002
Spanning tree enabled protocol rstp
Root ID Priority 32770
Address 0019.e7db.d100
Cost 4
Port 25 (GigabitEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32770 (priority 32768 sys-id-ext 2)
Address 0023.ea8d.cd00
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–
Gi0/1 Root FWD 4 128.25 P2p
Gi0/2 Altn BLK 4 128.26 P2p
What is the answer of Q.10,why it is C?I think B is the answer…I am confused.Can anyone tell and explain me…..
@Arsalan
The explanation already given for Q10 should be sufficient. I think where you might be getting confused is that the command to lower the BID uses the word priority, eg. spanng-tree vlan 2 priority 24576
Although this uses the word priority, it’s lowering the Bridge ID or BID, so the correct answer for Q10 is C
The option “F. The switch must be the root bridge for all VLANs on this switch.” Because, the stp used is RSTP and the root bridge is calculated by comparing Bridge priority/ Mac Address of all the switches in the domain.
And the “F” implies that This switch is root bridge for the vlans in its switch and it is wrong.
This Switch must be Root Bridge for all the VLANs in all the other switches in the domain.
So “A” Might be correct.
PLease correct me, if i am wrong
Hi ,
Anyone can explain to me how to get the default id for Qns 5
“The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768″ I cannot find 32768 in the diagram
There is no way to know the sw is the root bridge for all vlans in q5 so A is correct because rstp has 3 states discarding, learning and forwarding, in vce answer from spike has a mistake.. excuse my english
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Q13, I thouth the explanation is a bit confusing. It says;
“Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.”
I think this is wrong. What if Switch4 has lower MAC address, then will STP make two ports in switch4 as designated and block one of ports in switch3?
I think, switch3 ports will be desginated because for two segments between switch 3 and 4, the path cost to root is shorter from switch 3, that’s why it will make ports of switch3 as DP.
If switch4 has lower MAC address, this STP port selection doesn’t change.
So, going back to question assuming switch4 has lower MAC address than switch3, if 9tut explanation is true,
Switch4 port 2 will be root, 11 will be designated.
Switch3 port 1 will be designated, 12 will be blocked and alternate.
But if we only look at cost from segments to root, the answer stays the same.
Please correct me if I’m wrong.
why fa0/12 is inferior to fa0/1??! plzz helppp
Regarding Q13:
If port fa0/1 on Switch3 was connected with port fa0/11 (instead of fa0/2) on Switch4, then the port put on discarding status would be fa0/2 on Switch4???!!!
I think…yes…!!! Right???
Hello all friends..
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Disregard my previous comment!!!
Now I realize that it’s about Port ID (the lowest the better)…
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hello All:
I have big problem with STP/RSTP when it comes to choose which switch mac-address is the lowest one Is a known technique to figure this out? I mean I know to do that but during the exam we do not have time to make all these hexadecimal calculations we do not even have the resources to do so.
do you use a quick trick/astuce to do this 9tut? Please let me know I am taking the exam on friday.
Thank you all
A good under standing of base 16 is your friend. What is the highest what is the lowest. Evert\y mac address I have seen on exams has not needed to be converted but was obvious base on general knowledge of base 16.
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Q7 is clearly INCORRECT. After DTP is completed (trunks are negotiated), the very next thing to occur is election of the Root Bridge, which is STP, and the default Cisco equivalent of 802.1D is PVST+ (Per-VLAN STP, or an instance of 802.1D for each VLAN). If no other VLANs have been configured at this point, the election of the Root Bridge for VLAN 1 occurs. 802.1q does not come into play until after this, when and if other VLANs than VLAN 1 are configured. The correct answer is then C, 802.1D.
Q5 – A, C, E is indeed the correct one.
A is correct because RSTP consolidate port states into 3 states: Discarding, Learning, Forwarding – you can read about it here: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
F is NOT correct because RSTP in Cisco’s routers are actually a modified version that support PER VLAN RSTP. You can try it in Packet Tracer.
regarding Q13.
its fine that the mac address of sw3 is lower so one of the port of sw4 should be blocked.
bt i m confused which port index no should be referred? of sw3 or sw4 ?
they said index no of sending bridge i.e. sw3. i dint knw that. i have been taught to choose blocking port just by seeing index no of sw 4. i.e. f0/11 is higher thn f0/2 so f0/11 should be blocked.
is this theory correct ? pls help me out.
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